Why do i have a horrible smell in my basement?

I live in a house with a basement garage. When you walk down stairs there is a small area with 2 rooms, the furnace room and storage room and a door that leads out to the garage. Also there is a crawl space in the furnace room. I noticed a couple of months ago that it started to have a smell when you walk down the... show more I live in a house with a basement garage. When you walk down stairs there is a small area with 2 rooms, the furnace room and storage room and a door that leads out to the garage. Also there is a crawl space in the furnace room. I noticed a couple of months ago that it started to have a smell when you walk down the stairs and it only smells early morning and late at night. The smell has gotten worse , so we called the gas company to make sure it wasn t a gas leak. The guy said everything looked great but he said he could smell it to. It is a hard smell to explain except when you smell it , it hits you in the back of the throat and makes you gag. What could it be and who would you call to figure out what it is?
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